FIG. 1 is a diagram showing a known structure of a semiconductor device in which current loads are arranged in a matrix. The semiconductor device is finding various applications. In FIG. 1, a semiconductor device 200 comprises a plurality of data lines 202 in a parallel arrangement, a plurality of scanning lines 203 in a parallel arrangement running in a direction perpendicular to the data lines 202, and a matrix of current load cells 201 set at the intersections of the data lines 202 and scanning lines 203, respectively. The data lines 202 are voltage-driven or current-driven by a voltage driver or a current driver 230. The scanning lines 203 are driven by a scanning circuit 240. Examples of the semiconductor device include an organic EL (Electro Luminescence) display device in which organic EL elements being current loads are used as the current load cells 201.
There are two main driving methods for the semiconductor device, in which the current loads are arranged in a matrix, as follows:
(1) passive drive by which the lines are selected one by one, and the loads are driven only for a selected period of time; and
(2) active drive by which the lines are selected one by one, the value of current is memorized by memorizing information for driving the loads during a selected period of time, that is, a voltage corresponding to the value of current fed to each current load, and thereby the loads are driven with the memorized current value until next time the same line is selected.
A passive driving device is formed of current loads. For example, as shown in FIG. 2 (a), the current load cells 201, which are arranged in a matrix, may be realized from a simple structure with only a plurality of the data lines 202, a plurality of the scanning lines 203, and current loads 206 each being connected between the respective data lines 202 and scanning lines 203. In the passive driving device, however, since the loads are driven only for a selected period of time, a large current flow is required. Consequently, in the case of the passive driving device, the current loads 206 take heavy loads instantaneously, which may cause a problem with the reliability of elements that form the current loads 206. Moreover, the passive driving device consumes a measurable amount of power because of a drop in efficiency.
On the other hand, in an active driving device, the current load cells 201, which are arranged in a matrix, includes a plurality of the data lines 202, a plurality of the scanning lines 203, the current loads 206, and current load driving circuits 207 each of which is connected with the current load 206 between the data line 202 and the scanning line 203 for memorizing a voltage corresponding to the value of current fed to each current load 206 to drive the load as shown in FIG. 2 (b).
The current load driving circuit 207 in the respective current load cells 201 is made of a transistor or the like. The current load cell 201 has a complex structure as compared to that of the passive driving device. Nevertheless, the active driving device requires a small load driving current and the load on the current loads is reduced since they are driven for a long period of time from when a line is selected to when next time the same line is selected after all lines are selected. In addition, the active driving device consumes lower amounts of power because of its high efficiency. For the reasons mentioned above, the active drive may be superior to the passive drive in the load on the current loads and electric power consumption.
The structure of the current load driving circuit 207 for the active drive is broadly classified into two types: in one type (referred to as “voltage write-in structure”), a voltage to be applied by a semiconductor device (voltage driver 230 in FIG. 1) that feeds the respective current load driving circuits with voltage is memorized, and the respective loads are driven by a current corresponding to the voltage memorized; and in another type (referred to as “electric current programming structure”), current is applied by a semiconductor device (voltage driver 230 in FIG. 1) that feeds the respective current load driving circuits 207 with current, a voltage corresponding to the current is memorized, and the loads are driven by a current corresponding to the current.
Taking an organic EL display device as an example, it is often the case that current is memorized in an organic EL element of each picture element or pixel, and the current load driving circuits are formed of polysilicon Thin Film Transistors (abbreviated to “p-Si TFT”). Incidentally, since the p-Si TFT (obtained by low-temperature p-Si process) has high field effect mobility, it is possible to integrate parts of peripheral circuits or drivers with the display substrate, which enables high-speed and large-current switching control.
There has been disclosed in Japanese Patent Application laid open No. HEI5-107561 the voltage write-in structure as shown in FIG. 3 (see FIG. 7 of the Patent Application). A one-pixel display section 210 comprises: a light emitting element 220 whose one end (anode terminal) is connected to a power supply line 204; a TFT (Thin Film Transistor) 211 formed of polysilicon n-channel MOSFET, whose drain is connected to the other end (cathode terminal) of the light emitting element 220, and whose source is connected to a ground line 205; a hold capacitance 212 connected between the gate of the TFT 211 and the ground line 205; and a switch 213 placed between the gate of the TFT 211 and a data line 202. A control line K 215 is connected to the control terminal of the switch 213, and ON/OFF control is carried out based on a control signal K 215 transmitted through the control line K 215 (hereinafter a control line and a signal transmitted on the control line will be designated by the like reference numeral). When the control signal K 215 becomes active and the switch 213 is turned on, the hold capacitance 212 is charged by the voltage of the data line 202. At the same time, the voltage of the data line 202 is applied to the TFT 211 as a gate voltage, thereby turning on the TFT 211. Consequently, the current path of the power supply line 204, the light emitting element 220 and the ground line 205 is allowed to conduct, and the light emitting element emits light. The brightness or luminance of the light emitting element 220 is changed according to the gate voltage of the TFT 211.
However, with the p-Si TFT, there are considerable variations in the current capacity of respective transistors, and therefore, it is highly likely that the driving current differs between TFTs even when the same voltage is used. In this case, variations are produced in the brightness of the organic EL elements, and display accuracy deteriorates.
In order to solve the problem, there has been proposed, for example, in Japanese Patent Application laid open No. HEI11-282419 the electric current programming structure as shown in FIG. 4 (see FIG. 1 of the Patent Application). With this structure, effects are produced only by relatively small variations in the current capacity of TFTs in adjacent areas, and high-precision display can be achieved.
Referring to FIG. 4, in this circuit, one terminal of the switch 213 in FIG. 3, not the one being connected to the gate of the TFT 211, is connected to the gate of the TFT 216 (current conversion element) formed of polysilicon n-channel MOSFET, whose gate and drain are connected (i.e. diode-connected) to each other and whose source is connected to the ground line 205. Besides, the drain of the TFT 216 is connected to the data line 202 through the switch 214, and the control terminals of both the switches 213 and 214 are connected to the control line K 215. The control signal for controlling the brightness of the organic EL element is fed to the data line as a variable control current. The TFT 216 converts the current input into a voltage via the switch 214.
However, a current driver employed for the electric current programming structure needs an output circuit for supplying current to respective data lines so that it can simultaneously supply current to the respective current load driving circuits on the selected line through the data lines during a one-line selection period. Consequently, it is necessary to provide the current drivers as many as all the data lines, which drives up costs.
In addition, there is another problem in that contact points between the current drivers and a device having current load cells for active drive arranged in a matrix increase, which reduces reliability and productivity.
Furthermore, it has been considered to form the voltage drivers or current drivers with the p-Si TFT as well as a matrix of organic EL elements and current load driving circuits on the same substrate so as to reduce the number of parts and costs. In this case, however, yields, reliability and productivity decrease because the device as a whole increases in circuit size or scale as the circuit scale of current driver part becomes larger.